Manufacture of fusible plugs



July 14, 1942. G, M BOOTH 2,289,334

MANUFACTURE OF FUSIBLE PLUGS Filed DBG. 30, 1940 2 Sheets-Shegf. l 4

14 I 14 j; 5 2a 0 28 15 a 16 L Z6 60 21 v Z 25 4 INVENTOR ATTORNEYS July 14, 1942. G. M. BOOTH MANUFACTURE OF FUSIBLE PLUGS Filed Dec. 30, 1940 2 Sheets-Sheet 2 Harm/rm raw 1mm? Z 7 w L M w n m U lllllllll I/IIIII/ FZ m R MM kl Tail M m i 5 W0 4 4 l 8 g 3 Ja I a W K a 8 1 7 5 7 Patented July 14, 1942 UNITED STATES PATENT OFFICE MANUFACTURE OF FUSIBLE PLUGS Application December 30, 1940, Serial No. 372,365

13 Claims.

This invention relates to the manufacture of fusible plugs, e. g., plugs of the type having a fusible core which melts out at a predetermined temperature to release the contents of the vessel in which the plug has been inserted. Plugs of the character stated are generally used in pressure vessels, for example, where it is desired to relieve an internal pressure upon an excessive heat rise; ordinarily such plugs are employed as safety devices and should be inexpensive so that when melted out, they can merely be. thrown away. Fusible plugs are thus used in steam boilers, for example, and also in gas pressure cylinders such as those employed for the transportation of liquefied gases, although it'will be understood that plugs made according to the present invention may be applied to many uses, and are, in general, useful wherever fusible plugs of prior types have been employed.

Various difficulties have arisen heretofore in the manufacture of fusible plugs, and although some compensation for defects in the fusible alloys and for unreliable filling procedures has been obtained by adopting special shapes for the cavity or bore in which the fusible metal is cast, the difficulties have not, so far asI am aware, been completely or sufiiciently overcome in actual manufacturing practice.

For example, although the usual practice of cleaning the plug cavity by pickling provides a good surface for adherence of the fusible metal, great care and thoroughness must be used in rinsing the acid-cleaned surfaces in order to avoid subsequent failure of the plug by corrosion.

Another source of corrosion, which is apt to cause leakage or other failure of the structure, arises from remnants (in and about the cast core) of the fiux ordinarily employed to facilitate adherence, experience unfortunately revealing that the most effective fluxes seem to be those which are most corrosive. Among the prior methods of casting the alloy in the plug opening, one procedure has been to pour in the molten metal from a ladle, while another expedient has been to heat the plug body and press into it a quantity of the fusible alloy in wire form so that the heat of the body melts the wire. In such methods, it is sometimes possible for a skilled operator to float off considerable excess flux by due care in puddling, but experience indicates that this practice places too great reliance on human skill for consistent production of good plugs, and particularly to avoid the retention of quantities of flux in the core.

Further difficulty has arisen from the fact that the cast fusible metal shrinks considerably on cooling. In some cases attempts to avoid cavities or insufficiencies due to shrinkage have consisted in trying to produce a convex surface at the top of the molten metal when the plug is filled; but this practice does not appear to take care of the shrinkage problem properly, and indeed one relatively costly expedient has been to make the plug body extra long at the filling end so that the defective core surface and adjacent body portion may be later machined off to produce a neat appearance. It has been found that the tendency of the molten metal (particularly as evidenced at its upper surface in the plug opening) to shrink or form a central recess is a function of the rate of cooling-i. e., the faster the cooling,

the greater the temperature difference between the outside and center of the fusible metal. On the other hand, the corrosion resistance of the cast core (not only resistance to impurities left in the core but also to corrosive materials that may be contained in the vessel where the plug is later used) is greatly improved by a fine grain structure, and to provide such structure rapid cooling is necessary. Indeed, when cooling of sufficient rapidity for that purpose is employed, solidification takes place quickly near the plug body and progresses toward the center, with correspondingly progressive shrinkage, so that there is often produced a pipe or other void in the center of the core, or sometimes even a completely concealed void or cavity covered only by metal of insufficient strength or corrosion resistance.

Accordingly, important objects of the present invention are to provide for the manufacture of improved fusible plugs and specifically to avoid or minimize one or more of the difficulties such as have heretofore been encountered. More specifically, certain objects of the invention are: to provide filling procedures and apparatus for producing plugs having uniformly good adherence between the fusible alloy and the body, and for producing plugs which are uniformly free from voids, pipes, blow-holes or impurities such as may weaken the plug or may promote corrosion or other deterioration therein; to provide a procedure which may eliminate the necessity of pickling or other corrosive cleaning agents, e. g., by utilizing a plug body having a cavity readily cleanable by reaming and yet so processed and filled as to insure satisfactory adherence and strength; to provide for the manufacture of fusible plugs, wherein the problem of shrinkage is satisfactorily solved, and wherein the solidified core has a clean, attractive, smooth surface so as to require no subsequent finishing operation; and, furthermore, to afford desirable control of grain structure and particularly to provide, in a satisfactory manner, for very rapid cooling which contributes to fine grain structure and corresponding high resistance to corrosion.

Still further objects include the provision of improved apparatus and procedure for making fusible plugs, particularly the simplest types, in a rapid, inexpensive, and yet fully satisfactory and reliable manner.

Although the procedure and apparatus of the present invention will afford substantial improvement in the manufacture of a wide variety of fusible plug structures, including many conventional types now made, the invention may be conveniently illustrated by reference to a single and relatively simple type of plug. For example, such plug may comprise a metal body, e. g., of brass, bronze or iron, having a tapered male thread on one end, and a hexagonal head on the other, and having a tapered (i. e., frustoconical) hole through the center lengthwise of the axis, the large end of the hole being at the threaded end of the plug. Plugs of this character are intended, for example, to be screwed into a pressure vessel from the outside so that the pressure is exerted on the larg end of the cast-in fusible slug or core.

To the hereinabove stated and other ends, important features of my improved procedure include first cleaning (preferably by reaming alone) and then iluxing the internal opening or bore of the plug body, and then closing the open ends of the bore or opening so as to provide a t,

mold for casting the fusible alloy, but providing controllable arrangements for both inlet and outlet of molten metal with respect to the plug cavity. Referring further to the procedur now preferred, the plug body is then heated to the proper casting temperature as by a circulating heated fluid which comes in direct contact with the outside of the plug, and when pre-heating is complete molten fusible metal is run into the plug opening, e. g., to fill it. The molten metal is supplied in considerable excess, so that it not only fills the cavity but fiows vigorously through and out, and there is in effect a substantial flushing of the bore or cavity by the molten metal, removing not only excess flux and other foreign matter but also any trapped air, or gases such as vaporized fiux.

Further operation in the preferred procedure is to close the outlet from the filled cavity, and to solidify the metal rapidly by circulation of a cooling fluid around the outside of the plug body, the cooling fluid being advantageously so directed that the fusible alloy starts to freeze from an end or part remote from, e. g., opposite, the point or place where molten metal is supplied. During such procedure, the supply of molten metal, advantageously under pressure, is maintained and further metal is thus in practice added to continue the filling during cooling, and thus to compensate for or replace the loss by shrinkage. Finally, when the entire contents of the plug body are congealed, the supply of molten metal is discontinued and the plug is finished.

By way of example, the annexed drawings show, in somewhat simplified form in certain respects, certain advantageous apparatus for carrying out the procedure. Referring to the drawings:

Fig. 1 is a vertical section of the plug-holding aasase i and filling device, showing a plug in place for filling;

Fig. 2 is a section on line 2-2 of Fig. 1; and

Fig. 3 shows the structure of Fig. 1 in elevation, and shows diagrammatically the attendant operating and supply arrangements.

For purposes of illustration, it may be conveniently assumed that the apparatus is being used for filling a fusible plug with an alloy having a melting temperature somewhat lower than 200 F. (Paravanos alloy being an instance of many such), for example as in the plugs used in chlorine, freon, and other cylinders, where it is desired to have the plug melt out at about F. For heating purposes in apparatus of the character shown, and in making such plugs, it has been found that water at about 200 to 210 F. is a satisfactory fluid. Hence, in the following specific description reference will simply be made to hot water.

Referring to the drawings, the apparatus includes a stationary head I0 having a short cylindrical downward projection II which is conveniently plane-surfaced and of such dimension as to provide a closure for the inner or larger end of the opening I2 in the fusible plug body I3 to be filled. The plug body I3 is retained against lateral movement by fingers or prongs I 4 projecting down from the head If), and the lower or smaller end of the opening I2 is closed by a corresponding fiat-surfaced cylindrical projecticn IE on the upper side of a movable head I8, which is mounted upon and moved vertically by a rod I7, e. g., so as to clamp the plug bod 13 between the heads I0, I6, and with the bore or opening I2 thus closed. Preferably closures II and I5 are hardened andoi material to which the fusible metal will not adhere-for example, in the case of commonly used fusible metals, suitably repellant materials are aluminum alloys, hard rust-resisting steel, and the like. Advantageously, the clamping pressure between the heads I8, I6, is such that the ends of the plug body around the opening I2 are slightly indented by the projections I I, I5.

A cylindrical casing 20 i disposed about the mounted plug I3 and conveniently around the movable head I5 to provide a jacket for circulation of heating and cooling media. The casing 20 is carried by an annular head 2i which is mounted upon a tube 22 surrounding but free of the rod I! so that vertical movement of the tube 22 will displace the housing up into the position shown, or down therefrom for insertion and removal of plugs. To provide a fluid-tight seal a suitable gasket 24 is disposed between the upper flanged end of the casing 20 and the adjacent end surface of the head i0, and suitable packing 25 is contained in an appropriate recess in the annular member ZI for engagement with the surface of the rod IT. The casing 20 also has an inlet port or conduit 20 at a lower part thereof and a similar outlet conduit 27 at the top. The movable head I S conveniently has an annular flange or baffle 28 around its upper end, provided with suitable passages or ports 30, the arrangement being such that water, for example, enters through the conduit 26, circulates through the ports 30, past a substantial portion of the underside of the plug I3, and then up around the sides of the plug and finally out through the conduit 21.

The face of the stationary closure I I is provided with a pair of relatively small openings or ports 3|, 32, through which, respectively, fusible metal casing 43. The valve seats 33, 31 are respectively provided with valve members 45, 41, at the end of corresponding operating stems 49, which are moved (through suitable packing as at 52) from the exterior of the apparatus for opening or closing the valves at desired times.

For simplicity of illustration, the associated supply and operating structures are shown in a more or less diagrammatic form in Fig. 3. Thus, to a hose fill, carried by the input conduit or nipple 26, hot or cold water may be supplied by selective closing or opening of valves 6|, 62 in the hot and cold water supply pipes 53, 64; or the casing 23 may be effectively emptied by closing both valves 3| and 62 and opening the otherwise closed valve 55 in an extension of the pipe 66 (which connects the hose (ill with the valves 6| and 52) below the hose 6E). Discharge of circulating water from the casing may be through a hose El attached to the outlet 21. Through a suitable branch pipe it, hot water may also be supplied from the pipe 63 to the upper end of casing 4|,

whence as previously described, it circulates down into the head it and back up through the casing 43, say to a discharge pipe The source of molten metal under pressure generally designated 12, e. g., the source of fusible alloy, communicates with the pipe 35, and the molten alloy discharge pipe 39 may similarly lead to a receiving tank or sump 15.

Although other operating devices may be employed, such as screw, lever or other mechanical means, for the valves and movable parts of the casting device, and although such elements and the valves of the water supply system may be automatically operated in accordance with the pre-set programs, the depicted structure is shown to include, for simplicity of illustration, a plurality of simple but effective fluid-operated arrangements, for actuating the several elements of the casting device. Thus, for example, the spring-restored diaphragm devices 80, 8| (conveniently identical with each other) have their diaphragms respectively connected to the valve stems 5|, 49, for closure of the valves upon the admission of fluid under pressure to the outer surfaces of the diaphragms. Similarly, the tube 22 and the red I? may be operated by the springrestored piston devices 82, 83 (of identical construction), having their pistons respectively connected to the tube and rod for upward displacement of each of the latter upon admission of fluid under pressure to the lower piston surfaces. Connection between the tube 22 and the piston rod 84 of the device 82 is conveniently by-passed around the device 83 by a frame 85; and it will be understood that suitable valve or other means (not shown) may be employed to control the admission of fluid under pressure (e. g., oil or water) to, and discharge of fluid from, the operating chambers of the devices 83, 3|, B2, 83, through their illustrated connecting conduit. Thus with the structures shown, the valves may be operated at desired times and the tube 22 and rod likewise displaced to open and close the filling device and permit insertion and clamping, or removal, of plugs.

An example of casting procedure employing apparatus of the character shown in the drawings, is as follows: The tube 22 and the rod I! are moved downwardly to open the filling device and permit insertion of a plug body (with its bore previously reamed and fluxecl) within the prongs |4. Thereupon the rod I7 is moved up so as to lock the plug closure [5 against the lower end of the plug, with the upper end seated against the upper closure II, and the casing 20 is brought up into position around the plug body, all as shown. With the parts in this position, and the valves 45, 4! closed, valve 6| is opened and hot wateris caused to circulate in through the conduit 26, around the plug body, and out through the conduit 21, for a suificient time to heat the plug body and adjacent parts to the casting temperature, 1. e., above the melting point of the fusible metal. At the same time, it will be understood that hot water is also preferably circulating through the sleeves 4| and i3, so as to heat the molten metal ducts in similar fashion and likewise the head ID of the filling device.

With all parts suitably heated, the valve stems 5| and 43 are moved outwardly so as to open their respective valves 47 and 45. Molten metal flows into and fills the opening of the plug body, and overflows through the pipe 39. Circulation of molten metal in this fashion into and out of the plug bore is continued for a suitable time, thereby fiushing excess flux, air, vaporized fiux, and other impurities out of the plug bore. When removal of all unwanted foreign matter has thus been assured, the valve stem 5| is operated to close the outlet valve 41. Valve BI is closed, and valve 62 is opened, substituting cold water in the circulation within the casing 23. As explained hereinabove, the resulting rapid cooling action is first applied to the lower end of the plug and the metal is frozen at that point first. The inlet valve 45 for the molten fusible metal is kept open all this time, so that as freezing proceeds upward in the plug bore, further fusible metal is supplied through the valve seat 33 to make up for the shrinkage which ordinarily takes .place during the rapid cooling operation. After the passage of sufficient time for complete freezing of the metal in the plug bore, the inlet fusible metal valve 45 is closed and the circulating cold water may be turned off by closing valve 62. Valve 65 is open to drain water from the casing 20, and the latter and also the rod Il' may be lowered to release the now filled plug. It will be understood that the continuing circulation of hot water through the sleeves M, 43, and in the interior of the head l0, effectively prevents freezing of the fusible metal in and about the valve seats, although it does not interfere with complete freezing of the plug body.

The finished plug may now be simply removed and insofar as there may be small amounts of metal frozen slightly within the apertures 3|, 32 of the head, the resulting small projections or stems may be broken off as the plug is removed or may be subsequently taken off if they come away with the plug.

The finished article is completely filled With metal and affords a particularly satisfactory and,

reliable plug; Both ends of the castv core have a generally fiat appearance, and their surfaces are neat, and flush or substantially flush with the ends of the plug body. Moreover, with procedure such as described, there are no voids, cavities or depressions, either visible or invisible, by reason of the shrinkage of the metal in cooling, or by reason of trapped air or other gases. Foreign matter, particularly flux or other corrosive material, has been flushed out and there is nothing to prevent the cast metal from adhering strongly to the walls of the bore for a long time. It will thus be seen that the procedure and apparatus are simple to operate and afford extremely satisfactory results in the manufacture of the described articles.

It will be understood that the invention is not limited to the specific embodiments herein shown or described, but may be carried out in other forms without departure from its spirit as as defined by the following claims.

I claim:

1. A method of filling a fusible plug having an opening consisting of a straight-sided solid of revolution, comprising cleaning the opening by reaming, applying to the opening a flux having a boiling point below casting temperature of the fusible metal to be used, heating the plug body to a temperature not less than the melting point of the fusible metal, and while keeping said body heated, passing molten fusible metal into and out of said opening in quantity in excess of that suflicient to fill the opening and thereby flushing flux and impurities out of the filled opening, closing the opening against escape of fusible metal, and while maintaining a supply of molten metal under pressure in communication with one end of the opening, chilling the plug progressively from the opposite end to provide in the opening an adherent core of solidified fusible metal having a fine grain structure.

2. Apparatus for filling the core opening of a fusible plug, comprising means engageable with the plug for closing the ends of the core opening, associated metal handling means including means extending to a predetermined point of the core opening for introducing molten metal into said closed opening and means for withdrawing molten metal from said opening, and means for circulating fluid about the plug when engaged by the first-mentioned means, including fluid-directing means for initiating said circulation of fluid at a portion of the plug remote from the aforesaid point.

3. Apparatus for filling the core opening of a fusible plug, comprising plug engaging and filling means including means for closing the core opening of a plug body, means for introducing molten metal into said opening and passage means for discharge of molten metal from said opening, whereby said opening may be flushed and filled with molten metal, means for circulating cooling fluid about the plug as retained by said first-mentioned means, to solidify the metal in the opening, and means for closing said passage means, whereby further molten metal may be introduced by the aforesaid means therefor to compensate for shrinkage of metal solidifying in the core opening.

l. Apparatus for filling the core opening of a fusible plug, comprising means for introducing molten metal under pressure into the opening of the plug, plug-engaging means for closing the opening to permit the latter to be filled with molten metal by the first-mentioned means, means for cooling the plug while engaged by said second-mentioned means, to solidify the molten metal in the opening, and means including means for heating the first-mentioned means, whereby the first-mentioned means may continue to introduce metal to make up for shrinkage of previously introduced metal during operation of the cooling means.

5. Apparatus for filling the core opening of a fusible plug, comprising vertically spaced closure heads adapted to engage and close the opposite core opening ends of a plug body, means for holding said heads in closing engagement with said plug ends, and molten metal handling means including means for introducing molten metal under pressure into the core opening of a plug engaged by said heads and means for withdrawing molten metal from the upper end of the core opening of the plug engaged by said heads, whereby the core opening may be flushed and filled with molten metal.

6. Apparatus for filling the core opening of a fusible plug, comprising plug closing and retaining means adapted to hold the plug body with the core opening in substantially vertical position and including closure means for the lower end of the core opening, closure means for the upper end of said opening and having a plurality of apertures for passage of molten metal, and means for supplying molten metal under pressure to one of said apertures, whereby upon filling the 'core opening, excess molten metal may be discharged through another of said apertures.

7. Apparatus for filling the core opening of a fusible plug, comprising plug-engaging means for closing the core opening of the plug, means extending to a predetermined point of the core opening of the plug held by said engaging means, for supplying molten metal to said core opening, and means for circulating cooling fluid about the plug, including fluid-directing means for defining said circulation to cool the plug progressively to the aforesaid point of metal supply, from a portion of the plug remote from said point.

8. Apparatus for filling the core opening of a fusible plug, comprising a pair of spaced heads having substantially parallel surfaces adapted to engage and close the opposite open ends of a plug body, means for moving one of said heads relative to the other to engage and'hold a plug body therebetween with the opening closed, a casing movable over one of said heads and into a position to surround the space between them, conduit means associated with said casing for circulation of fluid therethrough and about a plug body held between the heads, means including passage means extending through one of said heads and a valve in said passage means, for controllable supply of molten metal to the core opening of the plug body, and means including passage means extending through one of said heads and a valve in said passage means, for controllable discharge of molten metal from the plug opening.

9. The apparatus of claim 8 wherein the heads are spaced vertically and the supply and discharge passage means extend through the upper head, and wherein are provided chamber means for circulation of heating fluid about the molten metal supply and discharge means, including means directing said circulation through a portion of said upper head to maintain the passage means therethrough in heated condition.

10. The apparatus of claim 8 wherein the supply and discharge passage means extend through the same head; wherein the casing extends from a place adjacent said head to a place substantially beyond the other head; wherein the conduit means include a pair of conduits entering said casing at points respectively adjacent said places, and wherein there is provided fluid directing means establishing a fluid path from the conduit adjacent the second-mentioned place, past end portions of the plug about the opening closed by said other head, and thence along the sides of the plug to the other conduit.

11. A method of filling a fusible plug having a reamable opening consisting of a solid of revolution formed by revolution, about an axis, of a line free of portions reentrant in a direction outwardly from said axis, comprising cleaning the opening by reaming, heating the plug body to a temperature not less than the melting point of the fusible metal, and while keeping said body heated, passing molten fusible metal into and out of said opening in quantity in excess of that sufficient to fill the opening and thereby flushing impurities out of the filled opening, closing the opening against escape of fusible metal, and while maintaining a sup-ply of molten metal under pressure in communication with one end of the opening, cooling the plug to provide in the opening an adherent core of solidified fusible metal, said cooling step including reducing the plug temperature first at a part remote from the point of maintenance of supply of molten metal to the plug opening and reducing the temperature, last, at said point of molten metal supply.

12. A method of filling the core opening of a fusible plug with molten fusible metal, comprising heating the plug body to a temperature not less than the melting point of the fusible metal, and while keeping said body heated, passing molten fusible metal into and out of said opening in quantity in excess of that sufiicient to fill the opening and thereby flushing impurities out of the filled opening, closing the opening against escape of fusible metal, and while maintaining a supply of molten metal under pressure in communication with one end of the opening, chilling the plug progressively from the opposite end to provide in the opening an adherent core of solidified fusible metal having a fine grain structure.

13. Apparatus for filling the core opening of a fusible plug, comprising plug closing and retaining means adapted to hold the plug body and to close opposite ends of the core opening, said closing and retaining means including movable closure means for one end of the core opening, stationary closure means for the other end of the core opening and having a plurality of apertures for the passage of molten metal, and. means for moving the movable closure means toward the stationary closure means to effect closure of the ends of the core opening of the plug disposed between said movable and stationary closure means, means for supplying molten metal under pressure to one of said apertures, whereby upon filling the core opening, excess molten metal may be discharged through another of said apertures, said stationary closure means being disposed with said second mentioned, discharge aperture substantially above the lowermost part of the core opening in the retained plug, to facilitate flushing by the excess molten metal, and means for closing said second mentioned, discharge aperture, to afford maintenance of supply of molten metal under pressure from the supplying means to the core opening through the first mentioned aperture after excess molten metal has been discharged.

GEORGE MARTIN BOOTH. 

